Volcano monitoring also involves the recording and analysis of volcanic phenomena not visible to the human eye, but measurable by precise and sophisticated instruments. These phenomena include ground movements, earthquakes (particularly those too small to be felt by people), variations in gas compositions, and deviations in local electrical and magnetic fields that respond to pressure and stresses caused by the subterranean magma movements. Some common methods used to study invisible, volcano-related phenomena are based on: 1. Measurement of changes in the shape of the volcano—volcanoes gradually swell or “inflate” in building up to an eruption because of the influx of magma into the volcano’s reservoir or “plumbing system”; with the onset of eruption, pressure is immediately relieved and the volcano rapidly shrinks or “deflates.” A wide variety of instruments, including precise spirit-levels, electronic “tiltmeters,” and electronic-laser beam instruments, can measure changes in the slope or “tilt” of the volcano or in vertical and horizontal distances with a precision of only a few parts in a million. 2. Precise determination of the location and magnitude of earthquakes by a well-designed seismic network—as the volcano inflates by the rise of magma, the enclosing rocks are deformed to the breaking point to accommodate magma movement. When the rock ultimately fails to permit continued magma ascent, earthquakes result. By carefully mapping out the variations with time in the locations and depths of earthquake foci, scientists in effect can track the subsurface movement of magma, horizontally and vertically. [Illustration: Scientist, wearing asbestos gloves and gas mask, samples volcanic gases from active vent.] 3. Measurement of changes in volcanic-gas composition and in magnetic field—the rise of magma high into the volcanic edifice may allow some of the associated gases to escape along fractures, thereby causing the composition of the gases (measured at the surface) to differ from that usually measured when the volcano is quiescent and the magma is too deep to allow gas to escape. Changes in the Earth’s magnetic field have been noted preceding and accompanying some eruptions, and such changes are believed to reflect temperature effects and/or the content of magnetic minerals in the magma.